Many conventional combustion engines with a varying number of cylinders are subject, during operation, to free forces and/or free moments acting upon the combustion engine. Such free forces and moments lead to vibrations in the vehicle.

Free forces and moments can be expressed mathematically in terms of various orders.

Free forces and moments of the first and second orders are largely predominant.

Calculating the terms of the first and second orders usually enables free forces and free moments to be expressed with good accuracy, although in some cases free forces and moments of higher orders may cause noise and be significant.

Knowledge of the number of cylinders of the combustion engine and their positioning can be used for calculating the free forces and moments. Taking into account only the free forces and moments of the first and second orders, it can be determined mathematically that single-cylinder engines are acted upon by free forces of the first and second orders. Thus such engines are not subject to any free moments. Two- cylinder engines with cranks mutually offset by 180° are acted upon by free moments of the first order and free forces of the second order. Four-cylinder in-line engines are acted upon by free forces of the second order. Three-cylinder in-line engines and five- cylinder in-line engines are acted upon by free moments of the first and second orders.

In contrast, six-cylinder in-line engines can be designed to be fully balanced so that no free forces or free moments act upon them.

Free forces and moments of a combustion engine thus result in the occurrence of vibrations in the vehicle. A known way of eliminating free forces and moments is by so-called balance shafts which rotate in phase relative to the crankshaft of the

combustion engine so that a compensating moment is provided. Elimination of first- order forces and moments is achieved by rotating such balance shafts at the same speed as the crankshaft, and elimination of second-order forces and moments is achieved by rotating the balance shafts at double the speed of the crankshaft.

Installing such balance shafts is technically complicated. US 5 083 535 refers to an example of a device for eliminating the free moment of the second order on a five- cylinder in-line engine.

SUMMARY OF THE INVENTION The object of the present invention is to provide an arrangement and a method which provide good balance in a combustion engine in a vehicle in a simple and effective manner and at low cost.

The object indicated above is achieved with the arrangement mentioned in the introduction which is characterised by what is indicated in the characterising part of claim 1. Heavy vehicles usually comprise one or more units for powering various components of the vehicle. In most cases such units are not fully balanced but likewise apply free forces and/or free moments to the vehicle during operation. Fitting such an unbalanced unit in a suitable position relative to the combustion engine makes it possible to cause the unit's free forces and/or free moments to counteract the free forces and/or the free moments of the combustion engine. To this end it is possible to select a type of unit which is particularly suitable for compensating the imbalance on a certain type of combustion engine. An existing unit provided with suitable imbalance may also be used. The resultant value of the free forces and/or moments from combustion engine and unit can thus at least be reduced, thereby eliminating or at least reducing the vibrations in the vehicle which arise from the combustion engine's imbalance during operation. The fact that such a unit has in any case to be fitted in the vehicle means that balancing of the combustion engine can be provided without installing any extra equipment such as balance shafts. The vibrations arising from the combustion engine can thus be reduced in a simple and effective manner and at low cost.

According to a preferred embodiment of the invention, the unit is adapted to being run at the same speed as the combustion engine. Forces and moments of the first order have a value which varies periodically with the crankshaft angle of the combustion engine. For it to be able to provide counteracting forces and/or moments with a corresponding periodic variation, the unit needs to be run at the same speed as the engine. Alternatively, the unit may be adapted to being run at double the speed of the combustion engine. Forces and moments of the second order vary periodically with double the crankshaft angle of the combustion engine. For it to be able to provide counteracting forces and/or moments with a corresponding periodic variation, the unit needs in that case to be run at double the speed of the combustion engine. Where applicable, two units may be arranged so that one of them is run at the speed of the engine and the other at double the speed of the engine. Free forces and moments of both first and second orders may thus be eliminated. With advantage, the unit comprises a transmission by means of which the combustion engine is adapted to powering the unit. A transmission with a transmission ratio of 1: 1 or 1: 2 will always provide the unit with a correct speed which continuously follows the speed of the engine powering it. The transmission may also connect the crankshaft of the unit to the crankshaft of the combustion engine at a mutual rotational position such that the occurrence of free forces and/or moments is counteracted in an optimum manner in all rotational states.

According to a preferred embodiment of the invention, the unit is fastened to the combustion engine. In this case the unit and the combustion engine are in direct contact with one another so that any movements of the combustion engine are substantially immediately counteracted by movements of the unit. Free forces and/or free moments from the combustion engine can thus be effectively balanced without resultant vibrations of the vehicle. The combustion engine may be a five-cylinder in- line engine. Combustion engines with an odd number of cylinders are substantially always unbalanced. The invention is nevertheless applicable to most types of unbalanced combustion engines.

According to a preferred embodiment of the invention, the unit comprises balance means for varying the balance of the unit with the object of substantially optimum counteracting of the free forces and/or the free moments from the combustion engine.

Since a vehicle has a complicated dynamic structure, it is not obvious where the unit has to be fitted and whether it has to be run exactly in phase relative to the combustion engine if optimum elimination of vibrations in the vehicle is to be achieved. It is thus for example also possible by experimenting with interchangeable counterweights to arrive at an imbalance of the unit which results in optimum elimination of vibrations in the vehicle.

According to another preferred embodiment of the invention, the unit has only one cylinder. The crankshaft of a single-cylinder engine is acted upon by free forces of the first and second orders. Fitting such a unit in a suitable position relative to, for example, a five-cylinder engine makes it possible to reduce considerably the engine's free moments. The unit may be an air compressor. Substantially all heavy vehicles comprise air compressors to supply compressed air to, for example, the vehicle's brake system.

The object indicated above is also achieved with the method mentioned in the introduction which is characterised by what is indicated in the characterising part of claim 10. Fitting such a unit in a suitable position relative to the combustion engine makes it possible to cause the unit's imbalance to counteract the combustion engine's imbalance during operation. With such a method, the vibrations arising from a combustion engine's imbalance can be simply and effectively reduced. Any vibrations arising from the existing unit are also reduced.

BRIEF DESCRIPTION OF THE DRAWING An embodiment of the invention is described below by way of example with reference to the attached drawing, in which:

Fig. 1 depicts an arrangement for balancing an engine in a vehicle according to the present invention.

DETAILED DESCRIPTION OF AN EMBODIMENT OF THE INVENTION Fig. 1 depicts schematically a combustion engine 1, here in the form of a five-cylinder in-line engine. The combustion engine 1 is arranged in an undepicted vehicle. The combustion engine 1 comprises five cylinders 2 arranged in line, with movable pistons 3. Reciprocating movements of the pistons 3 in the cylinders 2 are converted via connecting rods 4 to rotary motion of a crankshaft 5. A schematically depicted unit in the form of a single-cylinder air compressor 6 is here fitted to one side of the combustion engine 1. The air compressor 6 comprises a cylinder 7 with a piston 8 which is arranged for movement and adapted to providing compressed air in a compressed air system of the vehicle during operation. The compressed air system may for example be used for applying the vehicle's brakes. The piston 8 of the air compressor is connected via a connecting rod 9 to a crankshaft 10. A transmission 11 connects the combustion engine's crankshaft 5 to the compressed air compressor's crankshaft 10. In the case here depicted, the transmission 11 comprises a first gearwheel 1 la arranged on the combustion engine's crankshaft 5, and a second gearwheel 1 lb arranged on the compressor air compressor's crankshaft 10. A gearwheel belt 11 c connects the first gearwheel 1 la to the second gearwheel 11 b. The transmission 11 has a transmission ratio 1: 2 so that the compressed air compressor 6 is run at double the speed of the combustion engine 1.

Most types of combustion engines are subject, during operation, to free forces and/or free moments acting upon the combustion engine. The magnitude of such free forces and free moments can be calculated mathematically for the various types of combustion engines on the basis of knowledge of the number of cylinders and the mutual positioning of the cylinders. The free force acting upon the crankshaft during operation can be expressed as the aggregate of a number of terms in a series. As the terms in said series decrease quickly in value, the force can be estimated with good accuracy as the aggregate of the first two terms of said series. The first term is referred

to as the free force of the first order, and the second term as the free force of the second order. A combustion engine's free force of the first order can be determined on the basis of the piston acceleration mass force of the first order and be expressed mathematically as Fi=moro cos (a). The free force of the second order can be determined on the basis of the piston acceleration mass force of the second order and be expressed mathematically as F2=mo'r co''cos (a). Fi is the piston acceleration mass force of the first order, F2 the piston acceleration mass force of the second order, mo the mass of the piston + one-third of the mass of the connecting rod, r the crank radius, co the angular velocity, X = r/1 (crank radius/connecting rod length) and a is the crank angle.

The free force of the first order acting upon a five-cylinder combustion cylinder 1 will be the aggregate of the piston acceleration mass forces Fi of the first order for the five cylinders 2. The free force of the second order acting upon the combustion engine 1 will correspondingly be the aggregate of the piston acceleration mass forces F2 of the second order for the five cylinders 2. The piston acceleration mass forces Fi and F2 of the first and second orders for the respective cylinders 2 may also result in free moments acting upon the combustion engine 1. The free moment of the first order is the aggregate of the moments to which the piston acceleration mass forces F1 for the respective cylinders 2 give rise. The free moment of the first order can be determined on the basis of knowledge of the piston acceleration mass forces F1 of the first order for the respective cylinders 2 and the distance a between the cylinders. The free moment of the second order can correspondingly be determined as the aggregate of the moments to which the piston acceleration mass forces F2 of the second order for the respective cylinders 2 give rise. The free moment of the second order can likewise be determined on the basis of knowledge of the piston acceleration mass forces F2 for the respective cylinders 2 and the distance a between the cylinders.

For a five-cylinder in-line engine 1 the aggregate of the piston acceleration mass forces Fi and Fa for the five cylinders will be nil as regards both the first order and the second order. There will thus be no free forces acting upon the combustion engine 1 during operation. However, the piston acceleration mass forces F1 and F2 give rise to free

moments of both the first and the second order. With the distance a between the respective cylinders 2, the free moment of the first order in such cases can be calculated mathematically as 0. 449 Fi a, and the free moment of the second order as 4. 98 F2 a. The total moment Mm acting upon the combustion engine during operation will thus be Mm = 0. 449 F1 a + 4. 98 F2 a. With a k value, i. e. the ratio between the crank radius and the connecting rod length, of around 0.3 it may be found that Fi is about three times greater than F2. Despite this, the free moment of the second order for a five-cylinder in-line engine 1 will be definitely greater than the free moment of the first order.

A single-cylinder air compressor 6 gives rise, during operation, to free forces of the first order and the second order. The free force of the first order can be expressed mathematically as m0 r 92 cos (a) and the free force of the second order as myr'c2', cos (a). The total free force Fc acting upon the air compressor 6 will thus be myr'c2cos (a) + mn r o) 2 X cos (a). Since the air compressor 6 has only one cylinder, i. e. a = 0, there will thus be no free moments acting upon the air compressor 6 during operation.

The combustion engine 1 is subject, during operation, to the free moment Mm which tends to rotate the combustion engine 1 about a central point 12 of the crankshaft 5.

The air compressor 6 is subject, during operation, to the free force Fc acting at a point 13 of the crankshaft 10. According to the present invention, the air compressor 6 is fitted at such a position relative to the combustion engine 1 that the free force Fc of the air compressor 6 counteracts the free moment Mm of the combustion engine 1. The free force Fc of the air compressor 6 thus acts at a distance from the point 12 about which the moment Mm acts. Suitable positioning of the air compressor 6 relative to the combustion engine 1 will result in a linear distance from the point 12 such that the free force Fc of the air compressor 6 provides a moment which results in substantially optimum counteracting of the free moment Mm of the combustion engine 1. However, the value of the free moment Mm varies with the angle of rotation of the crankshaft 5 of the combustion engine 1, and the value of the free force Fc varies with the angle of rotation of the crankshaft 10 of the air compressor 6. For the moment from the free

force Fc of the air compressor 6 to provide optimum counteracting of the free moment Mm of the combustion engine 1, the crankshafts 5,10 need to be in an optimum mutual phase relationship. The counteracting moment provided by the free force Fc may thus be caused to vary in value at different crankshaft angles in a manner corresponding to the variation of the free moment Mm.

As the free moment of the second order for a five-cylinder in-line engine is greater than the free moment of the first order, it is appropriate for the transmission 11 to have a transmission ratio of 1: 2. The air compressor's crankshaft 10 will thus be run at double the speed of the combustion engine's crankshaft 5. The free force F of the air compressor 6 at a suitable distance from the point 12 can thus result in a moment which counteracts the free moment Mm of the combustion engine, which is of the second order. The vibrations in the vehicle which arise from the free moment of the second order can thus be considerably reduced.

However, a vehicle is a complicated structure. It may therefore be difficult to determine by theoretical methods the position at which the air compressor 6 has to be fitted to provide optimum counteracting of the free moment Mm of the combustion engine 1. Moreover, a combustion engine gives rise not only to a tilting moment Mm but also a twisting moment. The positioning of the air compressor in a vehicle has therefore to be effected experimentally in order to arrive at the air compressor position which results in a minimum vibration level in the vehicle. The air compressor 6 may likewise by trial and error be provided with balance means of various magnitudes so that the value of the free force Fc may be such as to provide optimum counteracting of the free moment Mm of the combustion engine 1. Also, the mutual angles of rotation of the crankshafts 5,10 can be adjusted with a view to optimum reduction of the free moment Mm of the combustion engine 1 and hence of vibrations occurring in the vehicle. Using an existing unit, e. g. an air compressor 6, in a vehicle makes it possible for the combustion engine 1 to be balanced in a simple and effective manner and at low cost.

The invention is in no way limited to the embodiment described but may be varied freely within the scopes of the claims. The combustion engine 1 may be of substantially any desired kind in which free forces and/or free moments occur during operation. The air compressor 6 may be substantially any desired unit in which free forces and/or free moments likewise occur during operation. The transmission 11 between the combustion engine's crankshaft 5 and the compressed air compressor's crankshaft 10 need not comprise gearwheels 1 la, lib and a gearwheel belt I lc, and the transmission may be of substantially any desired design.